Patent GB-1180529-A (GB)
Title: Ferroelectric Ceramic Optical Retardation Devices
IPC: G02F1/055
Filed: 1968-08-21
Abstract: 1,180,529. Light modulators. UNITED STATES ATOMIC ENERGY COMMISSION. 21 Aug., 1968 [14 Sept., 1967], No. 40081/68. Heading H4F. [Also in Divisions C1, G4 and G5] A plane polarized light, optical retardation device comprises a hot-pressed, optically uniaxial, birefringent, ferroelectric ceramic plate 20, Fig. 1a, having a multiplicity of grains of 2 microns or less in size, wherein the plate has an initial uniform polarization parallel to the major surfaces thereof and means for varying the retardation of plane polarized light (polarizer 14) passing through selected locations e.g. location 25, of the plate by incrementally varying the orientation of the optic axis i.e. the ferroelectric domains, of the ceramic at the surface of the plate or by varying the thickness of the plate. The orientation of the ferroelectric domains may be varied by means of electrode pairs 22 and 24 and 26 and 28 arranged to generate switching fields separately between each electrode pair and parallel to the plane of the surface at which the electrodes are disposed. Area 25 may thus be poled or switched to align its optic axis in any of four distinct directions i.e. the directions represented by arrows 36 and 38 and the opposite directions. The ceramic plate may be similarly poled at the other surface of the plate e.g. electrodes 22<SP>1</SP>, 24<SP>1</SP>, 261 and 281, independently without affecting the electrical polar condition at the opposite surface of the plate provided the dimension of the poled area in the direction of the poling field is less than the thickness of the plate. With the two sets of electrodes there may be sixteen different and distinct combinations of electrical polar conditions for plate 20 i.e. the retardation device has a plurality of different stable retardation states for the incident polarized light which are detected by analyser 16 and photosensitive device 18. The light incident on device 18 comprises a component with the intensity of light emerging from analyser 16 which depends on the birefringence i.e. ferroelectric polarization states and a component with intensity due to incoherent scattering in the ceramic plate. A greater number of distinct output light levels may be obtained by positioning further ceramic plates with similar electrode configurations in line with plate 20 e.g. plate 21. The number of different optic axis orientations may be doubled by adding further sets of diagonally opposing electrode pairs, Fig. 3 (not shown). The ceramic plate may comprise barium titanate or lead zirconate-lead titanate. In a second embodiment, Fig. 4, the ceramic plate 90, disposed between crossed polarizers comprises a light shutter which is opened by applying a switching voltage across electrodes 91 and 92 (similar to electrode pair 22 and 24 in Fig. 1a) and is closed by applying a switching voltage between transparent electrodes 93 and 94 to return the plate to electrical polarization normal to its major surfaces as shown by arrow 95. The electrode configurations may be deposited or etched on a large ceramic plate along with suitable connecting electrodes in a matrix array, Fig. 5 (not shown), the plate being utilized as a memory array or an optical display. In a further embodiment, Fig. 6, the intensity of the component of polarized light passed by the analyser is varied by varying the thickness of the ceramic plate 60 arranged to have all the optic axes aligned in a given direction (preferably normal to the direction of the incident light) to provide maximum birefringence. The thickness of the plate may be varied by generating either standing or travelling surface waves by means of a signal generator 62, input and output combtype electrodes 64 and 66, arid output load 68. Further thickness variation of plate 60 may be produced simultaneously by generating a second travelling surface wave on the opposite surface of the plate e.g. by signal generator 72, input and output electrodes (not shown) and load 78, this second travelling wave being at right angles to the first travelling wave. The devices of Fig. 5 (not shown), and Fig. 6 may be coupled with a system of Fig. 1a to form video display and imaging systems, including computer readouts and television picture reception. An array or group of comb-type electrodes may be substituted for electrodes 64 and 66 and aligned to generate different travelling surface waves which may be varied in phase relationship.